The present invention relates generally to cardiac radionuclide imaging techniques and more particularly to new and improved cardiac radionuclide imaging techniques.
In the practice of human or veterinary medicine, radioactive substances are sometimes used to image various body parts and bodily functions. These radioactive substances may be administered to a patient in a variety of ways, e.g., by being injected, inhaled, ingested, instilled or the like. Images created by capturing or visualizing the radioactivity may be mathematically or otherwise processed or analyzed, depending upon the purposes and goals of the particular examination. This medical field is commonly referred to as “radionuclide imaging” or “nuclear medicine.”
Currently, radionuclide imaging is performed in a variety of different clinical and research settings. For instance, in the case of a patient who experiences chest pain when exercising, a myocardial scan of the patient's heart may be performed. Such a myocardial scan typically involves administering a radioactive substance into the bloodstream of the patient and then using the radioactivity of said substance to image the myocardium (heart muscle) while the patient is physically or pharmacologically stressed (and possibly at rest, too, for comparison) in an effort to determine whether the myocardium is receiving sufficient blood flow during exercise. If a coronary artery supplying blood to the myocardium is obstructed in some fashion, it will deliver the radioactive substance to the myocardium more poorly than it otherwise would. As a result, an area of the myocardium that has insufficient radioactive substance delivered to it may be visualized as defective with respect to the remainder of the myocardium. Another type of radionuclide imaging of the heart is a radionuclide ventriculogram (or multiple gated acquisition scan) and involves using radionuclide imaging to examine the blood within the lumen of the chambers of the heart.
Accurate interpretation of images depends upon obtaining, processing and creating images of the highest quality. Falsely positive examinations can lead toward unnecessary therapy or additional testing (one or both of which may be dangerous and/or costly) and away from the actual cause of a patient's problem. One way in which falsely positive interpretations of cardiac radionuclide images occur is that a portion of the heart is obscured from an imaging camera by a bodily structure positioned between the camera and the portion of the heart, said bodily structure preventing the radioactivity emanating from the heart from properly reaching the imaging camera. Such an obscuring of radioactivity emitted by the patient is typically referred to as “attenuation” in the field of nuclear medicine.
In cardiac radionuclide imaging, it is important to distinguish a genuine, anatomic or physiologic defect (representing disease) from an artifactual one, such as one caused by attenuation. A common location in the heart of error caused by attenuation is the inferior (and nearby, especially posteriorly) left ventricular myocardium. The diaphragm, a muscle positioned between the abdomen and the chest, has long and widely been regarded as the cause of this artifact because the inferior wall of the heart is adjacent to the diaphragm. Hence, such an artifactual “inferior wall defect” has typically been considered to be attributable to the diaphragm and is often termed simply “diaphragmatic attenuation.” Alternatively, a more recent explanation for artifactual inferior wall defects is that fluid in the stomach, and not the diaphragm, is responsible for attenuation. (The stomach is usually just a few millimeters from the heart, on the opposite side of the diaphragm.)
A modest degree of success in decreasing artifactual inferior wall defects has been achieved by altering the positioning of the patient's body for imaging. For example, with planar technique, improved images may be acquired with the patient upright or right lateral decubitus in position. With single photon emission computed tomography (SPECT), better images are seen with the patient prone or “prone decubitus.” Also, electronic gating helps to distinguish this artifact from true defect, and mathematical means exist to diminish the visual effect of such artifacts.
In any event, despite the modest gains in decreasing artifactual inferior wall defects achieved in the manner described above, artifactual inferior wall defects remain a serious and important problem in the general clinical practice of radionuclide imaging of the heart.
Another source of artifact in cardiac radionuclide imaging is radioactivity within the left lobe of the liver, the stomach or the bowel. Such radioactivity may be so intense or close to the heart as to obscure the relatively lesser amount within the nearby heart muscle, thereby preventing portions of myocardium from being visible in the images. In addition, computerized and other methods that are intended to “correct” or to lessen mathematically the effect of radiation from one or more of the liver, stomach and bowel may cause the scan's images to contain nearby areas of heart muscle that appear to have less radioactivity than they actually do.